Algae bloom in Gulf Of Mannar, Oxygen level rises

Marine organisms in the fragile Gulf of Mannar ecosystem can now breathe easy. Scientists say the algal bloom that caused a ‘major mortality’ of coral fish and other organisms in the calm waters of the bio-reserve has subsided.
However, more issues are waiting to be resolved, if the native eco-system in the 21 islands that form the biosphere are to be saved. Marine experts are surveying the entire biosphere to look for lurking dangers, after the unusual bloom of Noctiluca scintillans, the algal species, had a killer effect a few days ago.
Over 5,000 coral fish died on the seaward side of Vaalai Island, one of the 21 islands constituting the Gulf of Mannar Marine Bio-reserve (GoMBR), and both ‘minor’ and ‘medium’ mortality was noticed towards the coast. Major mortality means the death of over 1,000 fish in a one-kilometre radius. Organisms that died included eels, sea bass, hardy head, grouper, squirrel fish, silver bellies, parrot fish, rabbit fish and gobids, all slowmoving coral fish species. The crabs captured by the bottom set gill net were found moulted and some were dead. Scheduled species like sea turtles, sea cucumbers and a sea cow also died.
Analysis of the dead fish indicated a slight choking of their gills. A low level of oxygen and high level of ammonia could have triggered their death, according to the fisheries department’s report. Studies have revealed that the oxygen levels at the sea bottom was only 0.7 mg per litre compared to the required oxygen levels of 4.5 to
6 mg per litre in the gulf, basically because of the proliferation of the alga by nearly 50 times in every litre of water. One reason for the algal bloom could be the nutrient-rich waters of the gulf, but corals mainly survive in low-nutrient waters. The enriched nutrients are either the result of upward current or due to the mix of sewage let into the gulf by 20-odd town panchayats dotting the coast and the industrial pollution from the Thoothukudi town in the southern end of the biosphere, say scientists.
A study by the Fisheries College in Thoothukudi (Tuticorin) has revealed that the coastal population near the Gulf of Mannar has shot up by 35% between 1990 and 2008. Unlike in the Andamans, where the presence of the coral reefs is far away from the shore, the distance between in the Gulf of Mannar is a mere eight km. Better sewage management in town panchayats and stricter pollution control measures in Thoothukudi have to be taken up immediately, say scientists.